The present invention relates to a drive assistance apparatus for a vehicle having a mechanical main transmission for driving displacement members of the vehicle, the apparatus comprising a main source of fluid under pressure suitable for being driven by first drive means to deliver fluid under pressure, and at least one hydraulic motor suitable for being fed with fluid under pressure by the main source of fluid to drive at least one displacement member of the vehicle, said motor having a stator and a rotor suitable for rotating relative to the stator about an axis of rotation, the motor comprising a cylinder block which belongs to a first one of the two elements constituted by the stator and the rotor, which is disposed inside a case, and which presents a plurality of cylinder-and-piston assemblies disposed radially relative to the axis of rotation, together with a reaction member for the pistons which belongs to the second one of said two elements, the motor also having a main fluid feed duct and a main fluid exhaust duct suitable for being put into communication with the cylinders, and an auxiliary duct connected to an internal space provided inside the case around the cylinder block and suitable for being put into communication with a fluid return tank, the motor being suitable for adopting a xe2x80x9cworkingxe2x80x9d configuration in which the pistons co-operate with the reaction member and are suitable for generating outlet torque under the effect of the fluid delivered by the main fluid source, and also a xe2x80x9cfree-wheelxe2x80x9d configuration in which the pistons are maintained in the retracted position inside their cylinders so that the rotor is free to rotate relative to the stator without generating any outlet torque.
French patent No. 1 425 800 discloses an assistance hydraulic motor and relates more particularly to maintaining said motor in its free-wheel configuration. It thus proposes using the leak return duct of the motor, which corresponds to the above-mentioned auxiliary duct, to connect it temporarily to a source of fluid under pressure which, by causing a relatively high pressure to exist inside the case, enables the pistons to be urged in such a manner as to cause them to be retracted radially into the cylinder block. That document also states that to pass from the free-wheel configuration to the working configuration, it is necessary to disconnect the leak return duct from said source of fluid under pressure and to connect it to a fluid return tank.
French patents Nos. 2 426 812 and 2 504 987 propose using resilient return springs to hold the pistons in their retracted positions inside their respective cylinders, while the motor is in its free-wheel configuration. For passing to the working configuration, those documents state merely that in operation the cylinders are fed with fluid under pressure, thereby causing the wheels carried at the free ends of the pistons to come into contact with the cam against the resilient return force exerted by the springs.
Conventionally, a vehicle fitted with drive assistance apparatus of the above-specified type is normally driven solely by the first drive means, e.g. the wheels of a first axle themselves driven by a main transmission that is mechanical. It is only when the travel conditions of the vehicle require additional drive force over and above that provided by the mechanical main transmission (e.g. going uphill) or additional braking force (e.g. going downhill) that the assistance hydraulic motor is used. This can apply, for example, under difficult conditions, e.g. on a worksite or on slippery ground. Conventionally, the assistance hydraulic motor is designed to drive other vehicle drive members, for example the wheels on a second axle.
With existing apparatuses, starting from a situation in which the vehicle is driven solely by the mechanical main transmission, it is necessary to slow the vehicle down considerably, or even to stop it completely, prior to causing the assistance hydraulic motor to change over from its free-wheel configuration to its working configuration.
When the vehicle is being driven solely by its mechanical main transmission, the drive members that are coupled to the assistance hydraulic motor cause the rotor of said motor to rotate, which motor is then in its free-wheel configuration. When the pistons are caused to move out from their cylinders so as to place their free ends (possibly fitted with wheels) in contact with the cam, this contact takes place suddenly, giving rise to shocks between the pistons and the cam, which runs the risk of prematurely damaging the cam and the free ends of the pistons (the wheels), and also to successive banging noises which are disagreeable and which give the driver of the vehicle the impression that the equipment is of poor quality.
The present invention seeks to propose an apparatus to which changeover from the free-wheel configuration to the working configuration takes place under better conditions, thus making it possible to perform said changeover while the vehicle is traveling at a relatively high speed and while avoiding or greatly limiting any risk of premature wear of motor components and/or disagreeable banging noises.
This object is achieved by the fact that the apparatus comprises means for performing the successive stages of a sequence for causing the motor to pass from its free-wheel configuration into its working configuration, said sequence comprising an initialization stage in which the auxiliary duct is isolated from the fluid return tank and is connected to a xe2x80x9cservicexe2x80x9d duct which is fed with fluid under pressure such that a xe2x80x9cservicexe2x80x9d fluid pressure exists inside the internal space of the case, and in which the main feed and exhaust ducts are placed at a common xe2x80x9cfillingxe2x80x9d pressure which is less than the service pressure but greater than the pressure of the fluid return tank, the sequence further comprising, after the initialization stage, a clutch engagement stage during which the auxiliary duct is put into communication with the fluid return tank and during which the main feed and exhaust ducts are isolated from each other.
In the free-wheel configuration, the pistons can be held retracted inside the cylinders either by fluid pressure inside the case, or by return means such as resilient springs.
In the first case, the piston deactivation pressure can be equal to or substantially equal to the service pressure and it can be obtained by the leak return duct being connected to the service pressure source. In the free-wheel configuration, the main feed and exhaust ducts are conventionally connected to the fluid return duct.
Thus, in this first case, when it is decided to change over from the free-wheel configuration to the working configuration, the first condition for obtaining the initialization stage, i.e. obtaining the service pressure inside the case, is already acquired, and the initialization stage is implemented by placing the main feed and exhaust ducts at the filling pressure which is higher than the pressure of the fluid return tank.
When resilient return means such as springs are used for deactivating the pistons, the main ducts and the fluid return ducts are normally connected to the fluid return tank in the free-wheel configuration. Consequently, when it is decided to pass from said configuration to the working configuration, it is necessary firstly to connect the leak return duct to the service duct so as to cause the service pressure to exist therein, and secondly to place the main feed and exhaust ducts at the filling pressure.
Such resilient return means enable the hydraulic motor to be deactivated without requiring deactivation pressure to be permanently maintained in the circuit.
In any event, at the end of the initialization stage, the main feed and exhaust ducts are at the filling pressure while the space inside the case is at the service pressure which is slightly higher than the filling pressure. In this situation, when the auxiliary duct is put into communication with the fluid return tank during the clutch engagement stage, the pressure inside the case is caused to drop so that the common pressure in the main ducts quickly becomes greater than said pressure inside the case, thus causing the pistons to be extended simultaneously from their cylinders so that all of them make contact with the cam at the same time. The banging noises are avoided because the fluid pressure inside the case is reduced progressively, even though it is reduced quickly.
During the clutch engagement stage, the main feed and exhaust ducts can be isolated from each other at the same time as the auxiliary duct is put into communication with the fluid return tank, or slightly after said operation.
Generally, even when the main feed and exhaust ducts are isolated from each other, the pressure in the main exhaust duct is greater than the pressure of the fluid return tank since some minimum booster pressure is usually maintained in the feed and exhaust ducts. Thus, even though the main exhaust duct is placed at xe2x80x9clowxe2x80x9d pressure compared with the feed pressure, said xe2x80x9clowxe2x80x9d pressure is greater than the pressure of the fluid return tank (close to atmospheric pressure), such that even those pistons which are connected to the main exhaust duct tend to move out from their cylinders during the clutch engagement stage.
In an advantageous embodiment, during the clutch engagement stage, the main ducts are isolated from each other only after the auxiliary duct has been put into communication with the fluid return tank.
This guarantees that the free ends of the pistons (optionally fitted with wheels) are put quickly into contact with the cam, the speed at which the pistons are extended being substantially the same, regardless of whether the pistons are connected to the feed duct or to the exhaust duct.
In which case, in a first variant, the clutch engagement stage comprises a first step during which the auxiliary duct is put into communication with the fluid return tank while the main ducts remain at the filling pressure, followed by a second step during which said main ducts are isolated from each other while the first drive means are inactive, and by a third step during which said first drive means are activated so as to drive the main source of fluid under pressure.
In this first variant, the clutch engagement stage is performed in three successive steps. In particular, the main source of fluid under pressure is not put into operation until all of the pistons are already in contact with the cam. Thus, during the second step of the clutch engagement stage, the rotor of the motor is driven by the wheel(s) to which it is coupled, the piston being in contact with the cam, thereby giving rise to a difference in pressure between the main feed and exhaust ducts which are isolated from each other.
By subsequently activating the main source of fluid under pressure during the third step of the clutch engagement stage, it is possible to increase said pressure difference progressively and to enable the hydraulic motor to perform its role of providing assistance. In other words, during the second step of the clutch engagement stage, it is the vehicle drive members coupled to the hydraulic motor which act as a xe2x80x9cmotorxe2x80x9d while the hydraulic motor is operating as a pump.
In a second variant, the clutch engagement stage comprises a first step during which the first drive means are activated so as to drive the main source of fluid under pressure while the auxiliary duct remains isolated from the fluid return tank, followed by a second step during which the auxiliary duct is put into communication with the fluid return tank and during which the main ducts are isolated from each other.
In this second variant, the main source of fluid under pressure is put into operation first, prior to causing the pistons to be extended from their cylinders so as to come into contact with the cam. As soon as the main feed and exhaust ducts have been isolated from each other and connected to the main source of fluid under pressure, a significant pressure difference is obtained between the ducts of the cylinders connected to the main ducts (at least those connected to the main feed ducts) and the inside space of the case which is put at the pressure of the fluid return tank, thereby accelerating the displacement of the pistons in the direction for bringing them into contact with the cam.
In addition, this makes it possible to ensure that the main source of fluid under pressure is engaged with its drive motor (in general the engine of the main transmission) under conditions in which the main feed and exhaust ducts are still connected together, which means that said source of fluid under pressure does not apply any torque for the hydraulic motor or is not subjected to any braking torque therefrom. This makes it possible for the clutch to be simpler and less expensive than is necessary for a situation in which torque is already present when clutch engagement takes place.
In another embodiment, the clutch engagement stage comprises a first step during which the first drive means are activated so as to drive the main source of fluid under pressure while the auxiliary duct remains isolated from the fluid return tank, followed by an intermediate step during which the main ducts are momentarily isolated from each other, and then momentarily reconnected to each other, and by a final step during which the auxiliary duct is put into communication with the fluid return tank and during which the main ducts are isolated from each other.
The intermediate step of the clutch engagement stage follows activation of the mechanical drive means for the pump. By momentarily isolating the main ducts before the auxiliary duct is put into communication with the fluid return duct, and then putting the main ducts back into communication with each other, this step makes it possible to reduce the stresses to which the mechanical drive means of the pump used for feeding the motor are subjected during clutch engagement, and to avoid excess pressures in the main ducts. After this intermediate step, as in the preceding embodiment and its variants, there is a final step of the clutch engagement stage during which the auxiliary duct is put into communication with the fluid return tank and during which the main ducts are isolated from each other for a long duration.
Advantageously, in accordance with the invention, the means for performing the successive stages of the sequence for causing the motor to pass from its free-wheel configuration to its working configuration comprise valves controlled by command means themselves under the control of a control unit.
This control unit controls passage through the various stages of the sequence and the various steps of the phases. It can be constituted by dedicated electronic circuits, but it is advantageously implemented in the form of a control unit such as a processor, a microprocessor or a microcontroller. It then serves not only to control passage through the various stages of the sequence, but also to control the durations of said stages and the steps therein.
Under such circumstances, the means for controlling the valves are advantageously pneumatic control means.
A source of pneumatic pressure is generally installed on a vehicle that is fitted with apparatus of the invention for the purpose of providing the vehicle with braking. Conventionally, the compressed air pressure is used for releasing brakes which are otherwise held on in the absence of compressed air. The use of pneumatic means thus makes it possible to take advantage of a source of air under pressure that is already in existence on the vehicle. Furthermore, when the vehicle is moving, compressed air is put under pressure so as to release the brakes of the vehicle, so air under pressure is naturally available to keep the controlled valves in the desired positions, without loss of energy, even when the assistance hydraulic motor is in its free-wheel configuration.